Sayyiduna Abdullah bin al-Sa'ib (may Allah be pleased with him) said: I was present with the Prophet (peace and blessings be upon him) in the year of the Conquest. He (peace and blessings be upon him) led the Fajr prayer, and he took off his sandals and placed them on his left side.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4258
Sayyiduna Abu Hurayrah (may Allah be pleased with him) said that the Prophet (peace and blessings be upon him) said: "When one of you prays, let him not place his sandals on his right or on his left; if he places them on his left, they will be on someone else's right. Yes, he may place them on his left when there is no other person on his left; otherwise, let him place them between his two feet."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4259
Sayyiduna Abu Hurayrah (may Allah be pleased with him) said that the Prophet (peace and blessings be upon him) said: "When one of you prays and takes off his sandals, let him not cause harm to anyone with them; let him place them between his two feet, or pray in them."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4260